Organic acids salts, or the corresponding acids, are useful in the food, beverage, flavors and perfumery industries. The salt may be the preferred form for manufacture, with subsequent conversion to the acid as with organic acids produced for flavors and perfumery. In fermentation, the acid is commonly the fermentation product but the salt of such acid may be the desired end product. For example, citric acid salts, particularly the sodium salt, are suitable for use as chelators, flavor enhancers and buffers in pharmaceutical, food and industrial applications, where a higher pH is required than may be provided by citric acid. Potassium citrate is used in pharmaceuticals and in food products. These salts are typically prepared by neutralizing the organic acid solution with a base containing the appropriate cation, e.g. potassium hydroxide or sodium hydroxide.
Two principal acid recovery techniques which are used commercially at this time in conjunction with the fermentation of citric acid and salts thereof are solvent extraction as outlined in U.S. Pat. No. 4,275,234 to Baniel et al, and a lime-sulfuric acid process. Additional modifications of these recovery processes have been proposed in U.S. Pat. No. 4,994,609 to Baniel and Gonen and in EP 0 432 610 to Baniel and Eyal, respectively. EP 0 432 610 describes the concurrent production of citric acid and alkali citrates by subjecting the mother liquor, obtained from the crystallization of citric acid after the known lime-sulfuric recovery process, to extraction with a water-immiscible organic extractant that contains at least one organic amine and a liquid hydrocarbon. The resulting extract is separated and neutralized with an aqueous alkali citrate whereby an alkali citrate brine is formed. The extractant is recycled and the aqueous alkali citrate brine is processed for recovery of alkali citrate. The processing may comprise cooling, addition of alkali, e.g. gaseous ammonia and a combination of such operations. In the examples, ammonia is added to convert the mono basic citrate in solution to the dibasic citrate. On cooling to 40 degrees C in a crystallizer, dibasic citrate crystallized out. If desired, in addition to or in lieu of adding alkali in the course of the processing of the alkali citrate brine, it is possible to add alkali during the neutralization operation. Baniel and Eyal describe a procedure for preparation of aqueous solutions of alkali citrates from which alkali citrates can be crystallized.
In addition, the literature describes many other techniques for the purification citric acid from the impure fermentation broth. Among the literature references is published European Patent Application 0 167 957 owned by Hoechst AG which discloses a process for isolating water soluble acidic compounds by bringing a solution of the acid into contact with a weakly basic, adsorbent, ion exchange resin, preferably those containing tertiary amino groups, and then desorbing the acid with water and/or steam.
Offenlegungsschrift DE 3 502 924, owned by Benckiser GmbH, discloses a citric acid purification process involving membrane filtration, preferably ultrafiltration, together with adsorption of impurities on a non-ionic resin such as polystyrene or polyacrylamide and crystallization.
U.S. Pat. No. 4,851,573, to Kulprathipanja et al discloses a method for separation of citric acid from its fermentation broth by contacting the broth with a water-insoluble macroreticular gel of a weakly basic anionic exchange resin possessing tertiary amine functional groups or pyridine functional groups and a cross-linked acrylic or styrene resin matrix. The citric acid is desorbed by water or dilute sulfuric acid.
Other methods which may be used to purify citric acid fermentation broths include ion exchange, nanofiltration. U.S. Pat. No. 5,032,686, to Duflot et al and assigned to Roquette, Freres, discloses a method for the recovery of citric acid from a liquor containing the same, by successively: 1) putting the liquor containing the citric acid in contact with a cationic resin in the hydrogen form for a duration sufficient to reach an optimal adsorption (approximately 90%) of the acid; and 2) treating the resin by elution whereby the fraction of eluate rich in purified citric acid is recovered. A preferred eluant is water at a temperature higher than 40.degree. C.
Hubertus Juetten, in his thesis entitled "The Enhanced Crystallization of Dicarboxylic Acids in Electrolyte Solutions", Michigan State University, 1992, states in the abstract that generally, carboxylic acid fermentations utilize anaerobic conditions and base addition for pH control. The salts of carboxylic acids then produced must be converted to the free acids and recovered. He states that a process for the crystallization of carboxylic acids by means of salting-out with the electrolyte sulfuric acid has been developed. The work disclosed is primarily directed to the recovery of succinic acid from an aqueous solution of sodium succinate and sodium acetate. Juetten also discloses experiments on the salting out of citric acid, L-tartaric acid and DL malic acid from saturated solutions of pure acids in water by addition of sulfuric acid (page 25). No experiments were disclosed on the application of this process to fermentation broth. Juetten suggested that the salting out process could be used as a modification of the Berglund et al process (U.S. Pat. No. 5,034,105) which is described in the thesis, page 45, as a process for preparing a supersaturated solution of carboxylic acid from a fermentation broth. The anaerobic fermentation used to produce succinic acid operates optimally at pH's where salts of the organic acids rather than free acids are formed. Berglund provides an electrodialysis method whereby the mixed salt stream produced by such fermentation creates supersaturation in any system where the salt is more soluble than the acid. This process results in a mixture of two acids, succinic and acetic. Juetten provides a detailed proposal for the modification of the Berglund process with respect to the recovery of succinic acid from a mixture of sodium succinate and sodium acetate. Juetten does not discuss or suggest the use of his proposed process for the recovery of the organic acid salt. In addition, Juetten does not discuss or suggest the use of any base as a "salting-out" agent.
Methods for the purification of salts have also been proposed. U.S. Pat. No. 5,041,645, to Alon et al discloses the preparation and recovery of alkali metal citrates by the addition of an alkaline metal base or salt to citric acid solutions and recovery in solid form by the addition of a C1 to C5 alcohol to the solution to cause precipitation. The addition of alcohol is not necessary in the invention claimed herein.
U.S. Pat. No. 3,944,606 to Rieger et al discloses the extraction of alkali metal or ammonium citrates from citric acid processes including fermentation with a specific water-immiscible mixture of aliphatic amines and organic solvents and re-extraction of the resulting organic solvent mixture with an aqueous solution of an alkali metal hydroxide, carbonate or bicarbonate, ammonia or their salts.
U.S. Pat. No. 4,447,364 to Staal, discloses the preparation of a stable solution of aluminum citrate by combining a solution of aluminum chloride with a solution of citric acid with vigorous agitation. After formation of an aluminum citrate solution, sufficient alkali metal or ammonium hydroxide is added to increase the pH to 5.5 to 7.5. During addition of base agitation is continued, resulting in a solution of aluminum citrate.
The recovery process for salts claimed herein does not require organic solvent extraction or a chemical reaction.